1. Field of the Invention
The present invention relates to a shoe for use in wellbores as are typically utilized in oil and gas production.
2. Description of Related Art
In boring a region of an oil or gas well, a drill bit is typically mounted on the end of a “string”. The “bit” or cutting pieces can be mounted on a shoe, and together these guide a “string” such as tubing, casing or liner through the wellbore as it is formed.
Alternatively, after boring a region of an oil or gas well a string of tools and/or tubing can be run into the wellbore. As the string is run it can meet obstructions as it travels through the wellbore. These obstructions may be ledges which form from well material during boring, formation wash-outs, or debris formed by unstable sections of the wellbore wall collapsing. Such obstructions can result in the string jamming in the wellbore. To prevent or minimize the effect of these obstructions, a shoe is conventionally mounted on the lower end of the string to guide the string through the centre of the wellbore.
The principle features of a shoe are to provide a guide during insertion of a string or tubing while being capable of being “drilled out” when the string or tubing is in position within a wellbore. The drilling out is necessary to provide a throughbore for the passage of fluids or further tool strings beyond the position of the shoe. To aid drilling out downhole, the shoe typically comprises a nose portion made of a relatively soft material, such as aluminum, zinc or alloys thereof which can easily be drilled through. The nose portion is mounted, traditionally by a standard unified screw thread, onto a stronger annular body. A suitable material for the body would be steel. The body may be a sub which houses cutting elements such as reamers, or alternatively the body may be the leading edge of the string or tubing which is being guided by the shoe.
After drill out, assuming the drill out is ideally concentric, there remains a continuous cylinder of the nose portion material threaded to the body of the shoe. However, the tolerance for the thickness is small, less than 1 cm, and any deviation of the drill during the drill out, i.e., non-concentric drilling, results in a high wear rate at one or more points of the cylinder. This can result in sections of the cylinder being completely drilled away and this local breach allows the remaining crescent shaped shell to peel away from the body with relatively little effort. The only resistance to this detachment being the greatly reduced bend strength of the crescent. The crescent which falls away can become trapped within the bore or casing and result in catastrophic problems, as it may obstruct the bore and cause the well to be unworkable.